Gilbert mixing cells (such as cell or mixer 100 of FIG. 1) are commonly used in many radio frequency (RF) applications and circuits to perform upconversion and downconversion. A Gilbert mixing cell or mixer 100 comprises a switching core or switching quad 102 and a transconductance circuit 104. The switching core 102 is generally comprised of transistors Q1 to Q4 (which can be NMOS transistors), and the transconductance circuit 104 is generally comprised of a differential input pair of transistors Q5 and Q6 (which can also be NMOS transistors).
Looking to upconversion, as an example and as shown, in operation the mixer 100 a differential input signal IFP and IFM is converted to a differential radio frequency (RF) signal RFP and RFM. To accomplish this, input signal IFP and IFM are provided to the gates of transistors Q5 and Q6, and a corresponding signal from this transconductance circuit 104 is provided to the sources of transistors Q1 through Q4. Differential local oscillator signal LOP and LOM are provided to the gates of transistors Q1 through Q4 in the switching core 102 so as to generate the differential RF signals RFP and RFM from the drains of transistors Q1 through Q4.
Typically, the layout for switching core 102 is similar to the diagram of FIG. 1, namely, that transistors Q1 and Q2 are adjacent to one another and transistors Q3 and Q4 are adjacent to one another. Turning to FIGS. 2 through 4, an example of such a layout can be seen. As shown, transistors Q1 through Q4 are generally formed of a number of polysilicon gate electrode “fingers” 203 that separate source regions 205 and drain regions 204 (which are arranged in alternating patters). Gate dielectrics 208 (which can be, for example, silicon dioxide) are generally formed between the substrate 201 and the gate electrodes 203 with sidewalls 210 (which can also be formed of silicon dioxide) formed on either side of the gate electrodes 203. Vias 206 (which can be filled with tungsten) can electrically couple the source and drain regions 204 and 205 to other layers, while vias 212 can electrically couple gate electrodes 203 to other layers. These transistor pairs Q1/Q2 and Q3/Q4 are, as shown, also have a dummy region 202 formed therebetween. There may also be other dummy regions (not shown) and other dummy features, such as “ghost polysilicon fingers,” (not shown) that may be used for balancing.
Turning to FIG. 5 through 7, the couplings for the transistors Q1 through Q4 can be seen. As shown in FIG. 1, differential local oscillator signal LOP and LOM are provided to the gates of transistors Q1 through Q4. To accomplish this, different portions 508 and 510 of metallization layer 502 are coupled to the gates of transistors Q1 through Q4. As shown, portion 508 is coupled to the gates of transistors Q1 and Q4, while portion 510 is coupled to the gates of transistors Q2 and Q3. However, in order to make these couplings, a jumper (which is provided to enable portions 508 and 510 to cross) is included in portion 508. As shown, this jumper includes vias 506, which couples the metallization layer 502 to metallization layer 504. Another alternative for a jumper would be generally parallel lines formed in metallization layer 504 for each of portions 508 and 510. Additionally, as can be seen in FIG. 6, metallization layer 504 can also couple the sources of transistor pairs Q1/Q2 and Q3/Q4 together with portions 602 and 604. In FIG. 7, the couplings for the drains of transistors Q1 through Q4 can be seen. Here portions 706 and 708 couple the drains of transistor Q1 through Q4 together. As shown, portion 708 also includes a jumper which uses vias 704 and metallization layer 704.
A problem with this arrangement is portions 508 and 510 (of FIG. 5) are not separate from one another but, instead, cross. This crossing can create an interference (generally known as local oscillator phase and amplitude imbalance) for both upconversion mixers or downconversion mixers because the portions of the differential local oscillator signal LOP and LOM interfere with one another, and as frequencies increase, this interference is even more apparent. Namely, the crossing breaks the differentially of the differential local oscillator signal LOP and LOM causing feed-through (for transmitters) or self-mixing (for receivers) to arise. Further in complex mixing, where two such mixers are used in an I (in phase) and Q (quadrature) fashion, the problem of local oscillator phase and amplitude imbalance leads to reduction in image suppression (for transmitters) and image rejection (for receivers). Therefore, there is a need for a mixer with an improved layout that generally compensates for local oscillator phase and amplitude imbalance.